Our hypothesis is that altered circadian rhythms (melatonin, sleep/activity/illumination, prolactin, cortisol, thyroid stimulating hormone-TSH) characterize women with postpartum depression. This hypothesis is based on the testable theory that rapidly changing reproductive hormones (estradiol, progesterone, follicle stimulating hormone-FSH, luteinizing hormone-LH') during the postpartum period precipitate disturbances in the amplitude, phase (timing') or temporal organization of circadian rhythms that contribute to depressed mood in predisposed women. In particular, melatonin and prolactin amplitude and phase, and their temporal relationship, are altered. Identifying abnormalities in the regulation of circadian rhythms can serve as a basis for developing specific chronobiological or other treatment modalities in future studies. Postpartum major depression can have disabling effects on the mother and family and impair the growth and neurocognitive development of the child. Investigation into its pathogenesis, with the ultimate aim of developing new treatment strategies, is needed urgently, particularly given limitations on the safe and effective treatment options documented in lactating women. To assess whether alterations in amplitude, phase or temporal organization of circadian rhythms distinguish postpartum depressed patients (DP) from postpartum normal control (NC) women, we will examine plasma melatonin, the sleep/wake and illumination cycle, and prolactin, cortisol, and TSH in relationship to mood and reproductive hormones in 100 unmedicated postpartum women, 50 non-psychotic, unipolar women, whose onset of a major depressive episode (MDE) by DSM-IV criteria occurs during the first 3 months postpartum, and 50 healthy NC subjects, matched for age, postpartum week, and breast-feeding status. We hypothesize that the circadian rhythms of postpartum DP, compared with NC women, will exhibit abnormal amplitude, phase or temporal relationships. This study would serve as the basis of future studies in which treatments designed to correct these underlying abnormalities in circadian rhythms, and restore them to those of NC women, thereby would improve mood in DP.